The present invention relates to a pilot signal cancelling circuit suitable for use in a multiplex transmission system in which a sinusoidal wave is employed as a pilot signal.
In the commercial FM stereo broadcasting, as well known, a sinusoidal pilot signal of 19 KHz is contained in a FM stereo composite signal to assist a synchronous de-modulation. This pilot signal is used to generate a subcarrier-frequency switching signal necessary to carry out stereo demodulation on the side of receiver. This pilot signal is not necessary, however, on the output side of a stereo demodulating circuit which serves to recover left and right audio information signals from the FM stereo composite signal. This is the reason why a pilot signal cancelling circuit for removing the pilot signal from the composite signal is provided before the stereo demodulating circuit.
The pilot signal cancelling circuit is arranged to generate a cancel signal on the basis of the pilot signal contained in the FM stereo composite signal, which is 180.degree. out of phase with the pilot signal, and to combine the cancel signal with the composite signal to cancel the pilot signal. A conventional pilot signal cancelling circuit employs an LC resonant circuit to generate a sinusoidal cancel signal. Inductance and capacitance elements in the LC resonant circuit are subject to variation with temperature and/or time and must be readjusted accordingly. In addition, the change of resonance characteristic often causes the phase of cancel signal to be shifted, thus deteriorating the stability of cancel signal.